10 Diffusion in Cells

Key Idea: Diffusion is the movement of molecules from high to low concentration. Diffusion through plasma membranes can be facilitated by various transport proteins.

What is diffusion?

fDiffusion is the movement of particles from regions of high concentration to regions of low concentration.

Diffusion is a passive process, meaning it needs no input of energy to occur. During diffusion, molecules move randomly about, becoming evenly dispersed. fMost diffusion in biological systems occurs across membranes. Simple diffusion occurs directly across a membrane, whereas facilitated diffusion involves helper proteins. Neither requires the cell to expend energy.

Factors affecting the rate of diffusion

Concentration gradient The rate of diffusion is higher when there is a greater difference between the concentrations of two regions.

The distance moved Diffusion over shorter distance occurs at a greater rate than over a longer distance.

The surface area involved The larger the area across which diffusion occurs, the greater the rate of diffusion.

High

Concentration gradient

Low

If molecules can move freely, they move from high to low concentration (down a concentration gradient) until evenly dispersed. Net movement then stops.

Lipid soluble solutes Glucose Barriers to diffusion

Temperature Thick barriers have a slower rate of diffusion than thin barriers.

Particles at a high temperature diffuse at a greater rate than at a low temperature.

Carrier protein Inorganic ion

Channel protein

Simple diffusion

Simple diffusion across membranes can occur by molecules moving directly through the membrane without any assistance e.g. at the alveolar surface of the lung, O2 diffuses into the blood and CO2 diffuses out. Sometimes the rate of diffusion across the membrane is too low to meet the cell's needs for a particular molecule.

Facilitated diffusion by carriers

Carrier-mediated facilitated diffusion allows the transport of large lipid-insoluble molecules through the plasma membrane. Each carrier protein is specific to the molecule being transported. Carrier proteins allow molecules that cannot cross the membrane by simple diffusion to be transported into the cell. An example is the transport of glucose into red blood cells.

Facilitated diffusion by channels

Channel-mediated facilitated diffusion provides channels that allow inorganic ions to pass through the membrane by creating hydrophilic pores in the membrane, e.g. sodium ions entering nerve cells.

2. Compare and contrast facilitated diffusion and simple diffusion across a membrane:

3. Describe two properties of a membrane that would facilitate rapid diffusion:

4. Consider the two diagrams below. For each, draw in the appropriate box what you would expect to see after one hour.

5. Cells create and maintain concentration gradients to keep particles (ions etc.) moving in one direction. They do this by using molecules up or transporting them away. Use diagrams to help explain how a concentration gradient maintains movement in one direction and why net movement stops if the concentration gradient is lost:

Particle with diameter of 5 nm Particle with diameter of 20 nm After one hour:

Soluble particles placed in at high concentration

Container of water at 20° C

Partially permeable membrane with pores of 10 nm.